Many machines of this type have a boom which is in effect substantially fixed relative to the center vehicle section without the possibility of suspension movements. This arrangement however severely limits the length of the boom which is available and the speed of operation of the machine. Attempts have been made to provide an improved suspension arrangement which allows an increase in boom length and/or an increase in vehicle speed.
One solution for this problem is to provide significantly improved suspension for the center vehicle section so that the center vehicle section moves upwardly and downwardly at a relatively slow rate in response to changing the ground level thus reducing the shock loading to the boom. This technique is currently being used widely on liquid sprayers.
However in machines for spreading particulate material, the use of suspension of the vehicle is undesirable to allow increased loads and the only suspension is that provided by large "floater" tires.
As shown in U.S. Pat. No. 5,052,627 issued Oct. 1, 1991 of the present inventor and a brochure showing a commercial arrangement of the patented machine manufactured by Ag-Chem, an arrangement is provided in which each boom is individually pivotally mounted on the center vehicle section for upward and downward movement about a horizontal axis parallel to the direction of movement of the vehicle. Each boom is supported by an elongate cable having an inner end attached to the center section at a height above the boom downwardly and outwardly to a position on the boom spaced from the center section so as to communicate the loading from the boom upwardly and inwardly to the center section. Although the patent does not show the coupling of the support member to the vehicle, this is effected in the commercial machine by a generally horizontal sway bar connected to the center section by a pair of pivotal links which interconnects the two booms and provides suspension movement by swaying back and forth across the vehicle. This arrangement provides very little suspension in the vertical direction but simply allows the boom to remain at a required height despite tilting of the center vehicle section in a side-to-side direction. One problem with this arrangement is the difficulty of accommodating the sway bar at the tank.
The height of the boom in this arrangement is controlled by actuating a cylinder which is connected in the inclined support member so it can be moved in to raise the outer end of the boom and let out to lower the outer end of the boom. This arrangement also has the disadvantage that when the boom is moved to a folded transport position, the height of the boom can be different on different occasions so that the boom moves to different positions when folded and can miss the intended locking position.
Simple tension springs in the inclined cable do not operate satisfactorily since the springs do not absorb energy so that the booms, after a shockload, continue to oscillate and thus convert the single shock into a series of repeated smaller shocks which are equally damaging to the boom structure.
Another technique used on a liquid sprayer which has been proposed is that of mounting the two booms for common vertical movement on a slide arrangement which allows vertical suspension sliding movement of the booms relative to the vehicle. A gas bag spring operating in the vertical direction between the sliding boom assembly and the center section provides resilience for the suspension movement of the booms.
The demands of increased vehicle capacity, increased vehicle speed and increased spreading width have significantly increased loads on the booms so that breakage and damage to booms has become a limiting factor in the operation of such machines.